Marco Rubio and Ted Cruz came to Thursday night's debate armed with an avalanche of opposition research, and both quickly unloaded on their mutual rival: Donald Trump.

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Rubio and Cruz, the two senators remaining onstage in Thursday night's debate, both trailed Trump by significant margins in last week's contests in South Carolina and Nevada.

And so while both Senators are trying to knock each other out of the race, it was Trump who entered the Houston confab as the bigger target -- and his opponents held nothing back.

Rubio, who has been relatively reluctant to engage with Trump, tore into him almost immediately as the debate kicked off, noting that he has hired undocumented immigrants to work at his own companies. He went on to raise the issue of Trump University, a program Trump ran that has encountered legal trouble, and that Rubio dismissed as a "fake university." And, in a turn of the tables, he mocked Trump for repeating only the same several catchphrases instead of engaging in substance.

Meantime, Cruz, who has been a much more aggressive and direct Trump critic in recent months, was ready to recite a long list of donations Trump has made to Democrats over the years. He recited back lines of Trump's previous comments on health care. And he reminded Trump of his previous positions regarding intervention in Libya. And both Rubio and Cruz pressed Trump to release his tax returns.

Here's a look at some of the biggest research hits Cruz and Rubio dropped on Trump:

On hiring undocumented immigrants:

"You're only person on this stage that's ever been fined for hiring people to work on your projects illegally," Rubio said early on in the debate.

Trump went on to argue that Rubio's point related to an incident from decades ago.

"You lied about the Polish workers," Rubio said.

"Yeah, yeah, yeah. 38 years ago," Trump said.

"Oh, he lied 38 years ago," Rubio responded later in the exchange. "I guess there's a statute of limitation on lies."

On Trump's donations to Democrats:

Cruz repeatedly noted that over the years, Trump had donated to Democrats--and he was prepared to list many of the recipients.

"It's interesting, now Donald promises he will appoint justices who will defend religious liberty, but this is a man who for 40 years has given money to Jimmy Carter, to Joe Biden, to Hillary Clinton, to Chuck Schumer, to Harry Reid," Cruz said. At another point in the debate, he accused Trump of having "funded" the bipartisan Gang of Eight immigration reform measure.

On "Trump University":

Rubio called the program a "fake school... And you know what they got? They got to take a picture with a cardboard cutout of Donald Trump...That's what they got for $36,000."

Trump said that he has won "most of the lawsuits" associated with the program.

Cruz, too, came ready to fight on that issue: “It's a fraud case," Cruz said later in the evening. "His lawyers have scheduled a trial for July. If you want to think about if this man is the nominee, having the Republican nominee on the stand in court being cross-examined about whether he committed fraud — you don't think the mainstream media will go crazy on that?”

Trump downplayed the matter as an easily winnable civil suit.

On Trump's past positions on health care:

Cruz, the former solicitor general of Texas and an ex-Supreme Court clerk, was in prosecutor mode Thursday as he pushed Trump on his health care positions, making references to several past comments Trump has made on the issue.

"Donald, true or false, you've said the government should pay for everyone's health care," Cruz said.

"That's false," Trump replied.

"You've never said that?" pressed Cruz.

"No, I said it worked in a couple of countries..." Trump said.

Cruz continued, paraphrasing a previous Trump statement, "But you've never stood on this debate stage and says it works great in Canada and Scotland and we should do it here?"

The cross-examination continued from there.

"Did you say if you want people to die on the streets, if you don't support socialized health care, you have no heart?" Cruz continued.

"Correct. I will not let people die on the streets if I'm president," Trump said.

On Trump's inheritance:

“Here's the guy that inherited $200 million,” jabbed Rubio. “If he hadn't inherited $200 million, you know where Donald Trump would be right now? Selling watches.”

“No, no, no, that is so wrong,” Trump said. “We'll work on that. I took $1 million and I turned it into $10 billion.

On Trump's favorite lines:

Rubio was the one who got tagged as repetitive in a previous debate, but on Thursday, he mockingly rattled off Trump's most-repeated remarks.

“Everyone’s dumb, he’s going to make America great again. Win, win, win. He’s winning in the polls. And the lines around the states,” Rubio said, demonstrating again that he'd studied up on the current poll-leader. “Every night. Same thing.”